Chapter 1: Measurements and Units (copy)

  • Physical Quantity is a quantity that can be measured. It consists of a numerical magnitude and a unit.

SI Units

  • Are set systematic international units, for measuring mass, time, and other base units and derived quantities.
Mass
  • Is the measure of the amount of substance in an object.
  • Has the following effects:
    • all objects are attracted to the Earth. The greater the mass of an object, the stronger is he Earth’s gravitational pull on it.
    • all objects resist attempts to make them go faster, slower, or in a different direction. The greater the mass, the greater is the resistance to change in motion.
  • SI base unit is kilogram (kg).
Time
  • SI base unit is second (s).
  • Following are some of other units for second:
Length
  • SI base unit is metre (m).

 

Measuring Length and Time

Measuring Length

  • Ruler can be used to measure small distances of a few centimetres (cm).
    • They are able to measure to the nearest millimetre (mm).
    • Can measure upto one metre.
    • Example of usage:
    • height of the table
  • Micrometer is used to measure objects that are too small to be measured with vernier calipers
    • Gives a precision of 0.01 mm
    • Can measure measurements of less than 25 cm.
    • Example of usage:
    • diameter of a wire
  • Vernier Calliper is a useful tool for measuring both internal and external diameters.
    • They are able to measure to a precision of 0.01 cm.

Measuring Time

  • Time intervals of many seconds or minutes can be measured using a a stopwatch.
    • Some instruments have analogue or a digital display.

Volume and Density

Volume

  • It is the quantity of space an object takes up.
  • It’s SI unit is cubic metre (m^3).

Density

  • It is the quantity of mass per unit volume of a substance.
  • Formula:
    • Density= Mass/Volume
    • ρ=m/v
    • ρ is a greek letter ‘rho’ .
    • (both equations are same, written in different ways; i.e first one is the word equation and the other one is symbolic equation.
  • SI unit of density is kilogram per cubic metre (kg m^-3).
  • Objects that are less dense than water will float on water; and objects that are denser than water will sink.

Measuring Volume and Density

Measuring Volume

Liquid
  • Its volume can be measured using a measuring cylinder.
  • Most cylinders have scale marked in millilitres (ml) or cubic centimetres (cm^3).
Regular Solid
  • If an object has a simple shape, its volume can be calculated through formula’s. For example:
    • volume of a regular block = length x width x height
Irregular Solid
  • If the shape of the object is too awkward to calculate its volume, then immerse the object in a measuring cylinder filled with water and then calculate the increase in water level. This value will be the volume of the object.

Measuring Density

  • Once volume of the object is obtained through one of the above methods suitable, find the mass of the object using a balance.
  • Following is one of the example:

 

Checking the mass and density of a liquid

  • An easy method of finding the density of a liquid is by using a small float called hydrometer.

 

  • The relative density on a hydrometer indicates the relative density of the liquid.
  • Checks like these are important in some production processes.